The present invention relates to a high-speed optical memory device and an optical circuit using this device. Particularly, this device is used for an optical temporary memory device, light clock and optical operator in for the fields of the next-generation multimedia communications, and optical signal communications such as microprocessor interconnection and optical computer.
The demand for large-capacity bidirectional communication network has been recently increased and is now steadily growing. In addition, the means for the transmission of information among more people is going to be developed and ready for use on a national scale as a social capital to be prepared in the twenty-first century. In order to build up this basic technology for the new century, it is necessary to use information processing techniques by which a large quantity of bidirectional information can be processed at higher speed. A part of the conventional electronics has evolved to optoelectronics for optical technology and is steadily expanding to photonics for optical technology to all. In this optical information processing and communication, it is important to increase the speed of signal processing and the amount of signal to be processed per unit time.
According to a document of, for example, "Semiconductor Laser, Light Communication and Optical Device", Conference Report:ClEO/QELS, '93 Report III, by Yawara Kaneko, et al., the trend of recent light communication toward great increase of the amount of information to be processed can be divided into the following two types.
The first type is the wavelength division multiplexing (WDM) system, or optical frequency division multiplexing, in which light of a different wavelength is mixed in a light pulse of several microseconds through several nanoseconds, thereby to increase the amount of information per pulse.
In this system, a light signal is generated by swiftly changing the oscillation frequency of a semiconductor laser to comply with an electrical signal which is desired to be transmitted. In addition, the light path through which this light signal is transmitted is changed chiefly by an optical device using electrooptical effect. Therefore, the response time of optical media necessary for the modulation of light pulse depends on the processing time in electric signal circuits, and hence the optical device can be made of an existing semiconductor material of silicon or germanium.
However, since light rays of different wavelengths on a single pulse are transmitted over a long distance, the light signal waveform is disturbed by group velocity dispersion and the light amplification in a repeater base station becomes irregular. Thus, there is a limitation in the maximum amount of signals which can be multiplexed.
Therefore, a time division multiplex system for transmitting and receiving shorter light pulses at high speed has been investigated as the second type for processing a large amount of information per unit time. In this system, since short light pulses of several picoseconds are successively transmitted at constant intervals of time as a train of light pulses of light signal, there are no such waveform disturbance and irregular light amplification as in the wavelength division multiplexing system mentioned above. In addition, since digital signals can be easily processed, a large amount of signals can be correctly transmitted over a long distance at high speed without any error in reading.
In order that this system can be practically used, it is necessary to use optical devices such as higher-speed light signal generators, light-path switchers and light signal readouts, and an optical system in which these devices are arranged in parallel at high density to operate in synchronism with each other. In addition, these fundamental optical devices depend on optical media with a high-speed response.
These optical media can be achieved not by silicon used so far of which the response speed to light pulse is as slow as at most several nanoseconds, but by a low dimensional system material such as of semiconductor superlattice gallium arsenide of which the response speed is as high as a value from several hundred picoseconds to several tens of picoseconds. In addition, faster response media of several picoseconds or below have been realized by use of conjugated system organic compound such as polydiacetylene. New optical devices can be expected to be produced by the development of the technology for the amount of driving light and establishment of process.
This fast response characteristic is closely related to the nonlinear optical effect of substance. The nonlinear optical effect is the phenomenon that when intense light such as laser beam is irradiated on a substance, electronic polarization occurs within the substance in proportion to a high-order power, such as square and cube, of electric field intensity of light. The fast response characteristic due to the nonlinear optical effect of substance is achieved by the change of charge distribution within the substance irradiated with laser beam and the rapid generation of new polarization. In other words, if the electric field intensity of incident light and induced polarization of substance are represented by E and P, respectively, the relation between both can be given by the following equation: EQU P=x1E+x2E.sup.2 +x3E.sup.3 +. . .
where .chi..sup.(1), .chi..sup.(2), .chi..sup.(3) . . . are coefficients called first-, second-, third-, . . . order electric susceptibility, respectively. The effect associated with the second-order term and above is called the nonlinear optical effect. Since the coefficients are normally decreased with the increase of the order number, the effect cannot be confirmed without intense light beam. The nonlinear optical effect due to the even-order, or second-order, fourth- order, coefficients does not occur in center-symmetrical substance such as glass fiber because induced polarization is canceled out when the polarization units of atoms and molecules themselves of a substance and the arrangement thereof within the substance are symmetric with respect to the center. As a result of this polarization, new light is generated of which the frequency is different from that of the incident light, and the refractive index and absorption coefficient of the substance are changed in proportion to the amount of incident light.
In practice, the effect associated with optical switch is the third-order nonlinear susceptibility. The amount of incident light necessary to make the substance function activated can be decreased with the increase of the sensitivity value. In other words, if the refractive index of a substance before being irradiated, the amount of incident light and the refractive index at the time of being irradiated are n.sub.0, I and n, respectively, the relation among them can be expressed by the following equation: EQU n=n.sub.0 +n.sub.2 I
where n.sub.2 is the nonlinear refractive index, and can be expressed in the unit of cm.sup.2 /W by the third-order nonlinear sensitivity .chi..sup.(3) (esu unit) and the following equation. EQU n.sub.2 =1.6.times.10.sup.8 .pi..sup.2 .times.3/cn.sub.0.sup.2
The light passing through the substance can be switched from a light path to another, modulated and shut off by changing the refractive index, reflection angle, transmissivity and reflectance. Since this polarization remains within the substance until it is extinguished by light or heat, a fast-speed optical switch must be made of a material of which the polarization is swiftly extinguished immediately after the incident light is passed therethrough. This effect is increased with the increase of the amount of incident light as shown by the above equations. Thus, when part of the light transmitting through the substance is again incident to the substance or when a resonator mirror is mounted on the substance, to increase the amount of light within the substance by feedback and arrive at the extent that it exceeds a particular threshold, the light within the substance is increased so that an extreme switching effect appears. Conversely, when the incident light is gradually decreased after the optical switch is operated by a constant value of light and again arrives at the extent that it is less than the threshold, the light passing through the substance is suddenly decreased.
The presence of these two definite states of passing light bisected by the threshold is called the optical bistability. When these low transmissivity and high transmissivity in this optical bistability are used as the on-state and off-state, respectively, the two states can be switched at high speed by changing the amount of incident light. In other words, the substance of an optical bistable switch using the third-order nonlinear optical effect fundamentally depends on the sum of the third-order nonlinear optical susceptibity and light polarization. The on and off states can be more sensitively made with the increase of nonlinear refractive index n.sub.2. Also, the optical switch using the optical bistability phenomenon can be driven at high speed by the increase of the speed at which the polarization once established by light is extinguished. In the signal transmission processing system using actual light, since the switch is driven by a small amount of signal light, it is necessary that constant bias light be added to bring the operating point to around the threshold so that the amount of all input light is changed at around the threshold in accordance with the presence or absence of signal light.
As described above, the optical bistable switch using the third-order nonlinear optical effect is able to make high speed operation which could not be made by the conventional electric switch, and thus it is increasingly expected to be used for the large-capacity high-speed light communication in the next generation.
However, in the actual optical information processing system, so far the first information generation is made by a computer chiefly of conventional electronic circuits, and the signal is converted into optical pulses, or converted from electric signal to light signal, by a semiconductor light-emitting device and then transmitted through an optical fiber. Therefore, for large-amount light communication, it is necessary to connect a large number of electrooptical transducers in parallel and concentrate all the light from those transducers into a single optical fiber. In order that the light signals from the separate electrooptical transducers can be regularly arranged to be parallel, it is necessary to exchange constant synchronizing signals among the transducers. In addition, when short light pulses are treated, the synchronizing means must have the same precision as that of the pulse duration.
Moreover, not only in the simple light path switching using the above-mentioned optical switch, but also in various different light signal processing operations such as logic sum by use of a combination of a plurality of optical switches, it is required to provide means for synchronizing the operations of the optical switches. In other words, a special optical memory function is necessary to hold the light signals at the respective devices for a constant time and regularly transmit in synchronism with a reference clock signal.
The above-given optical bistable switch can be turned on by applying a light signal to exceed the threshold, but cannot be swiftly turned off at an arbitrary time. In other words, when the optical bistable switch is operated without bias light, it returns to the off-state after the signal light has been completely passed through the switch. Thus, it has no memory means. In addition, when the bistable switch is operated with bias light, the bias light itself is required to be once stopped at high speed. Therefore, the light source, or semiconductor laser itself must be turned on and off at high speed, and it cannot be operated at a high speed of picoseconds or below.
In addition, although the synchronization among a plurality of transducers has so far been controlled by an electric signal, it cannot be controlled at super high speed by a light pulse of several picoseconds or below. Thus, light communication cannot be performed which transmits super short light pulses at high density.
Moreover, a huge laser system must be provided in order to generate light pulses of the order of picoseconds or below necessary for the control by super high speed light signal, and there are no simple generator means.